1. Field of the Invention
This invention relates to the field of oil and gas production from subterranean reservoirs. More particularly, the invention is an improved method and apparatus for the determination of certain important properties of reservoir rock, including pore volume compressibility, porosity at stress and relative porosity change.
In the recovery of oil and gas from underground rock formations, certain characteristics and properties of reservoir rock are of importance in estimating the production potential of the reservoir. A characteristic of particular importance is pore volume, which is the aggregate void space within the rock. A related property is porosity, which is the ratio of the total pore volume to the bulk volume of such rock. The bulk volume is the sum of the total pore volume and the grain volume. Grain volume is the volume of discrete rock particles (grains), i.e., the volume of "solid" rock. Porosity is indicative of the oil and gas storage capacity within reservoir rock.
As oil and gas are withdrawn from an underground rock formation, the fluid pressure in the pores of the rock decreases. However, the external pressure on the reservoir rock, which is due, in part, to the weight of the overlying strata, remains constant. As the disparity between the external and internal pressure (the net overburden pressure) increases, pore compression occurs. As such, the porosity of reservoir rock--and therefore the oil and gas reserves of a reservoir--will vary with the net confining pressure. Pore volume compressibility is thus a measure of the production potential of a reservoir, and can also be used as to calculate hydrocarbon storage capacity of the reservoir at various production phases.
2. Description of Related Art
A number of techniques exist for measurement of the aforementioned characteristics. The standard reference for reservoir rock core analysis is API RECOMMENDED PRACTICE FOR CORE-ANALYSIS PROCEDURE by the American Petroleum Institute, API RP 40, 1960. PETROLEUM RESERVOIR ENGINEERING: PHYSICAL PROPERTIES by Amyx et al., Reissue Edition, at 36-64 (McGraw-Hill 1988) provides a discussion of reservoir rock characteristics and a useful comparison of measurement techniques.
A fluid expulsion procedure known as "liquid squeeze out" is currently the most commonly applied technique in the oil industry to measure pore volume compressibility. In this method, a core sample is saturated with either brine or oil, placed in a rubber sleeved core holder, and subjected to a simulated overburden pressure and a pore pressure. The net overburden pressure is increased stepwise by an increase in sleeve pressure or a decrease in pore pressure. Fluid, i.e., brine, is expelled from the pores as the net overburden pressure increases. The volume of expelled fluid is an indication of the pore volume reduction. Pore volume compressibility can then be calculated at any pressure according to the relation: ##EQU1## Where: C.sub.p =Pore volume compressibility, vol/vol/psi
V.sub.p =Pore volume, cc PA1 P=Net overburden pressure, psi
Successive readings must be taken to define the compressibility curve for a ,given specimen of rock taken by core sampling.
Another method for determining pore volume compressibility is described in U.S. Pat. No. 4,782,501. This method employs computed tomographic (CT) scanning based on X-ray attenuation measurements at various confining stresses.
Both methods suffer from the drawback that a number of successive measurements must be taken to define the compressibility curve for a given reservoir rock sample. Further, to the extent that a noncompressible fluid, such as brine, is used to saturate the core sample, the compressibility determined in the laboratory will be the result of a hydrostatic load which differs from the actual reservoir loading. Under actual conditions in the hydrocarbon rock reservoir, the contraction is only in the vertical direction which is referred to as unilateral or uniaxial stress. Therefore, the laboratory measured hydrostatic strain condition must be converted to the uniaxial strain condition.
Thus, there is a need for a more efficient and cost effective method and apparatus to calculate pore volume compressibility.